What lab tests are recommended for the diagnosis and management of gout?

Laboratory Tests for Diagnosis and Management of Gout

The definitive diagnosis of gout requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirate, with serum uric acid measurement being essential for ongoing management and monitoring of treatment efficacy. 1

Diagnostic Laboratory Tests

• Synovial fluid analysis: Identification of MSU crystals through polarized light microscopy is the gold standard for definitive diagnosis of gout 1

• Serum uric acid (SUA) level: While hyperuricemia supports the diagnosis, it is insufficient alone for diagnosis as many patients with hyperuricemia never develop gout, and some patients with acute gout may have normal SUA levels during an attack 1

• Renal function tests: Measurement of renal function (serum creatinine, estimated glomerular filtration rate) is essential in all patients with gout or hyperuricemia, as it impacts treatment decisions and dosing of medications 1

Management and Monitoring Tests

• Serial serum uric acid measurements: The target for urate-lowering therapy is SUA below 6 mg/dL (0.36 mmol/L) for most patients, or below 5 mg/dL (0.30 mmol/L) for patients with severe gout (tophi, frequent attacks) 1

• Liver function tests: Recommended when initiating allopurinol therapy, especially in patients with pre-existing liver disease 2

• Complete blood count: Monitoring for potential bone marrow depression, particularly in patients receiving allopurinol with other medications that might cause this adverse effect 2

• Cardiovascular risk assessment: Evaluation of cardiovascular risk factors is recommended in all patients with gout or hyperuricemia 1

Medication-Specific Laboratory Monitoring

• For allopurinol therapy:

  • Baseline renal function is essential as dosing must be adjusted in renal impairment 2
  • Regular monitoring of SUA levels to titrate dose until target is reached 2
  • Periodic monitoring of liver function and complete blood count 2

• For febuxostat therapy:

  • Baseline and periodic liver function tests 1
  • Regular SUA monitoring for dose adjustment 1

• For uricosuric agents (e.g., probenecid, benzbromarone):

  • Baseline renal function 1
  • Urinary pH (alkaline urine preferred to prevent crystal formation) 2
  • 24-hour urinary urate may be helpful in selected cases 2

Common Pitfalls in Laboratory Testing for Gout

• Relying solely on serum uric acid for diagnosis: SUA may be normal during acute attacks, and hyperuricemia alone does not confirm gout 1, 3

• Inadequate monitoring of SUA during treatment: Regular monitoring is essential to ensure target levels are reached and maintained 1, 2

• Failure to adjust medication dosing based on renal function: Particularly important for allopurinol, which requires dose reduction in renal impairment 2

• Not checking for drug interactions: Particularly important when using allopurinol with drugs like azathioprine, mercaptopurine, or anticoagulants 2

• Overlooking cardiovascular risk assessment: Patients with gout have increased cardiovascular risk that should be evaluated and managed 1

By following these laboratory testing recommendations, clinicians can accurately diagnose gout, appropriately select and monitor treatment, and reduce the risk of medication-related adverse events while optimizing outcomes for patients with gout.